1. Field of the Invention
The present invention relates to an EL (electroluminescence) display device formed with a semiconductor element (an element using a semiconductor thin film) built on a substrate, and to an electronic device comprising the EL display device as a display.
2. Description of the Related Art
Techniques of forming a TFT on a substrate have greatly advanced in recent years, and development of applications to active matrix type display devices is proceeding. In particular, a TFT using a polysilicon film has a higher electric field effect mobility (also referred to as mobility) than a TFT using a conventional amorphous silicon film, and it is capable of high speed operation. It therefore becomes possible to perform control of pixels by a driver circuit formed on the same substrate as the pixels, although conventionally by a driver circuit external to the substrate.
This type of active matrix display device obtains many advantages, such as lowered manufacturing cost, smaller display device, increased yield, and reduced throughput, by building various circuits and elements on the same substrate.
In addition, research on active matrix type EL display devices having EL elements as self-luminescing elements has become spirited. The EL display device is also referred to as organic EL display (OELD) or an organic light emitting diode (OLED).
The EL display device is a self luminescing type which differs from a liquid crystal display device. The EL element has a structure in which an EL layer is sandwiched between a pair of electrodes, and the EL layer is normally a lamination structure. A lamination structure of xe2x80x9ca hole transporting layer, a light emitting layer, and an electron transporting layerxe2x80x9d proposed by Tang, et al., of Eastman Kodak Co. can be given as a typical structure. This structure has extremely high light emitting efficiency, and at present nearly all EL display devices in which research and development is proceeding employ this structure.
Furthermore, additional structures such as a hole injecting layer, a hole transporting layer, a light emitting layer, and an electron transporting layer formed in order on a pixel electrode; or a hole injecting layer, a hole transporting layer, a light emitting layer, an electron transporting layer, and an electron injecting layer formed in order on a pixel electrode may also be used. An element such as a fluorescing pigment may also be doped into the EL layer.
A predetermined voltage is then applied to the EL layer having the above structure by a pair of electrodes, and recombination of carriers thus occurs in the light emitting layer, and light is emitted. Note that the light emitting of the EL element is referred to as driving the EL element throughout this specification.
In addition, note that a light emitting element formed by an anode, an EL layer, and a cathode is referred to as an EL element throughout this specification.
Roughly divided, there are four color display methods for an EL display device: a method of forming three types of EL elements corresponding to R (red), G (green), and B (blue); a method of combining white color light emitting EL elements and color filters; a method of combining blue or blue-green EL elements and fluorescing bodies (fluorescent color changing layers, CCMs); and a method of using a transparent electrode for the cathode (opposing electrode) and overlapping EL elements corresponding to RGB.
Generally, however, there are many organic EL materials with which the brightness of red color emitted light is low compared with the brightness of blue color and green color emitted light. The brightness of red color images displayed therefore becomes weaker when an organic EL material having these types of light emitting characteristics is used in an EL display device.
Further, a method of using orange color light having a slightly lower wavelength than that of red color light has been conventionally performed because the brightness of red color light is low compared to the brightness of blue and green light. However, the brightness of red color images themselves which are displayed by the EL display device is low, and when display of the red color image is attempted, it is displayed as orange.
In consideration of the above, an object of the present invention is to provide an EL display device for displaying an image which has a good balance between the desired brightnesses of red, blue, and green light, using organic EL materials with different brightnesses of red, blue, and green light.
The applicant of the present invention has made the voltage applied to an EL element which performs display of a color having a low light emission brightness higher than the voltage applied to an EL element which performs display of a color having a relatively high light emission brightness in an EL display device which performs time partitioning gradation display.
An EL driver TFT which performs control of electric current to an EL element has a relatively larger current flow than a switching TFT which controls the driving of the EL driver TFT in order to make an EL element emit light. Note that control of the driving of TFT means that an on state or an off state of the TFT is set by controlling the voltage applied to a gate electrode of the TFT. In particular, for the above structure the present invention has more current flowing in the EL driver TFTs of pixels displaying colors having low light emission brightness than in the EL driver TFTs of pixels displaying other colors. A problem therefore surfaces in that the EL driver TFTs of the pixels displaying colors having low light emission brightness degrade quicker than EL driver TFTs of pixels displaying other colors due to hot carrier injection.
The applicant of the present invention, in addition to the above structure, made the length of a lightly doped drain (LDD) region of the EL driver TFTs of the pixels displaying colors having low light emission brightness longer than the length of an LDD region of the EL driver TFTs of the pixels displaying colors having high light emission brightness as measurers measured against deterioration of the EL driver TFTs by hot carrier injection.
Note that, throughout this specification, length of the LDD region refers to the length of the LDD region in a direction which links a source region and a drain region.
Furthermore, at the same time the channel width(W) of the EL driver TFTs of the pixels displaying colors having low light emission brightness is made larger than the channel width(W) of the EL driver TFTs of the pixels displaying colors having relatively high light emission brightness.
Note that, throughout this specification, channel width(W) refers to the length of the channel region in a perpendicular direction to the direction which links the source region and the drain region.
Even though the amount of current for controlling the EL driver TFTs is increased by raising the voltage applied in accordance with the above structure, the present invention can suppress deterioration of the EL driver TFTs. In addition, it becomes possible to regulate the brightness of light emitted by the EL elements by the value of the voltage applied to the EL elements, and to display an image having vivid colors and good balance between the brightnesses of emitted light in red, blue, and green colors. Note that the present invention may employ not only the time partitioning gradation display but also the other method of the display.
The structure of the present invention is shown below.
In accordance with the present invention, an EL display device having a plurality of pixels, each containing an EL element, characterized in that:
the EL display device performs gradation display by controlling the amount of time light is emitted by the plurality of EL elements; and
a voltage applied to the plurality of EL elements differs in accordance with the color displayed in the plurality of pixels containing the EL elements, is provided.
Also in accordance with the present invention, an EL display device having:
a plurality of pixels, each containing
an EL element;
an EL driver TFT for controlling light emission from the EL element; and
a switching TFT for controlling the driving of the EL driver TFT;
characterized in that:
the EL display device performs gradation display by controlling the amount of time light is emitted by the plurality of EL elements;
a voltage applied to the plurality of EL elements differs in accordance with the color displayed by the plurality of pixels each containing the EL elements;
the plurality of EL driver TFTs are composed of n-channel TFTs; and
the higher the voltage applied to the plurality of EL elements, the longer the length, in the longitudinal direction of a channel, of an LDD region of the plurality of EL driver TFTs, is provided.
Further, in accordance with the present invention, an EL display device having:
a plurality of pixels, each containing:
an EL element;
an EL driver TFT for controlling light emission from the EL element; and
a switching TFT for controlling the driving of the EL driver TFT;
characterized in that:
the EL display device performs gradation display by controlling the amount of time light is emitted by the plurality of EL elements;
a voltage applied to the plurality of EL elements differs in accordance with the color displayed by the plurality of pixels each containing the EL elements;
the plurality of EL driver TFTs are composed of n-channel TFTs; and
the higher the voltage applied to the plurality of EL elements, the larger the width of a channel region of the plurality of EL driver TFTs, is provided.
Also in accordance with the present invention, an EL display device having:
a plurality of pixels, each containing:
an EL element;
an EL driver TFT for controlling light emission from the EL element; and
a switching TFT for controlling the driving of the EL driver TFT;
characterized in that:
the EL display device performs gradation display by controlling the amount of time light is emitted by the plurality of EL elements;
a voltage applied to the plurality of EL elements differs in accordance with the color displayed by the plurality of pixels each containing the EL elements;
the plurality of EL driver TFTs are composed of n-channel TFTs;
the higher the voltage applied to the plurality of EL elements, the longer the length, in the longitudinal direction of a channel, of an LDD region of the plurality of EL driver TFTs; and
the higher the voltage applied to the plurality of EL elements, the larger the width of a channel region of the plurality of EL driver TFTs, is provided.
In accordance with the present invention, an EL display device having:
a plurality of pixels, each containing:
an EL element;
an EL driver TFT for controlling light emission from the EL element; and
a switching TFT for controlling the driving of the EL driver TFT;
characterized in that:
the EL display device performs gradation display by controlling the amount of time light is emitted by the plurality of EL elements;
a voltage applied to the plurality of EL elements differs in accordance with the color displayed by the plurality of pixels each containing the EL elements; and
the higher the voltage applied to the plurality of EL elements, the larger the width of a channel region of the plurality of EL driver TFTs, is provided.
The present invention may also be characterized in that the amount of time light is emitted by the plurality of EL elements is controlled by a digital signal input to the switching TFTs.
The present invention may also be an electronic device employing the above EL display device.